High efficiency organic electroluminescent display and method for fabricating the same

ABSTRACT

An organic electroluminescent display has: anode electrodes of R, G and B unit pixels formed separate from each other on a substrate; organic thin-film layers of the R, G and B unit pixels formed on the anode electrodes; and a cathode electrode formed over an entire surface of the substrate. The anode electrode of at least one unit pixel, among the R, G and B unit pixels, has a thickness different from anode electrodes of the other unit pixels. The anode electrode of each of the unit pixels comprises a first film having a high reflectivity and a second film for adjusting a work function. The second film of at least one unit pixel, among the unit pixels, has a thickness different from the second films of the other unit pixels. The second film of the R unit pixel is thicker than the second films of the other unit pixels.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C. §119 from my applicationHIGH EFFICIENCY OELD AND METHOD FOR FABRICATING THE SAME filed with theKorean Industrial Property Office on 1 May 2003 and there duly assignedSerial No. 10-2003-0028076.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a flat panel display and, moreparticularly, to an organic electroluminescent display and a method forfabricating the same, capable of improving luminous efficiency and colorreproduction by differentiating thicknesses of anode electrodes ofrespective R, G and B unit pixels.

2. Prior Art

Generally, an organic electroluminescent display (OELD) is classifiedinto a front surface emitting display and a rear surface emittingdisplay depending upon the surface of luminescence of light. On thebasis of a substrate, in case of the rear surface emitting OELD, a lightfrom an electroluminescent layer is emitted and passes through thesubstrate, while in the case of the front surface emitting OELD, lightfrom the electroluminescent layer is emitted without passing through thesubstrate.

The efficiency of the rear surface emitting structure is determineddepending upon optical characteristics of a reflection film and atransmissive anode electrode, and electrical characteristics of anorganic thin-film layer including an electroluminescent layer. A holetransporting layer is formed so as to be thicker than an electrontransporting layer since maximum constructive interference in opticalcharacteristics is generated at a thickness of ¼ wavelength of lightemitted. The mobility of the hole transporting layer is faster than thatof the electron transporting layer in terms of the electricalcharacteristics. Therefore, the thickness of the electroluminescentlayer presenting the maximum efficiency is determined when a full colorOELD of the rear surface emitting structure is manufactured.

On the other hand, the thicknesses of the hole transporting layer, theelectroluminescent layer and the electron transporting layer locatedbetween a reflective anode electrode and a transflective cathodeelectrode for measuring the optical thickness, and the electricalthickness in the front surface emitting OELD, are determined differentlyfrom the rear surface emitting OELD.

There have been prior attempts to obtain the maximum efficiency and thehighest color purity by controlling the thicknesses of a hole injectingand transporting layer, an electroluminescent layer and an electrontransporting layer making up the organic thin-film layer interposedbetween the anode electrode and the cathode electrode. Japanese PatentRegistration No. 2846571 has disclosed technology in the area of therear surface emitting organic electroluminescent display capable ofobtaining a high color purity and efficiency by setting an optical filmthickness of the anode electrode, the cathode electrode and the organicthin-film layers between the anode and cathode electrodes to achieve apeak in the strength of light emitted from the electroluminescent layer.Further, Japanese Laid-open Patent Publication No. 2000-323277 hasdisclosed technology in the area of the rear surface emitting organicelectroluminescent display capable of obtaining a high efficiency andcolor purity by differently forming the thickness of thin-film layers,except for the electroluminescent layer, among the organic thin-filmlayers interposed between the anode electrode and the cathode electrode,depending upon the R, G and B unit pixels.

However, the front surface emitting organic electroluminescent displayhas a problem in that, although the thickness of the thin-film layers isset to ¼ wavelength of a desired light, it is difficult to obtain adesired efficiency and color purity since the electroluminescent layeris located between reflection sections of the reflective anode electrodeand the semitransmissive cathode electrode.

On the other hand, in the front surface emitting electroluminescentdisplay, U.S. patent application assigned Ser. No. 10/385,453 entitled“Organic Electroluminescent Device Employing Multi-Layered Anode”, byKwanhee Le, filed in the United States Patent & Trademark Office on the12^(th) day of Mar. 2003, has disclosed technology capable of improvingluminescence characteristics by forming an anode electrode of amulti-layered structure.

Anode electrodes of the respective R, G and B unit pixels are formed onan insulating substrate. The anode electrodes include a first anode anda second anode. A pixel defining layer is formed to expose portions ofthe anode electrodes, thereby forming apertures of the respective R, Gand B unit pixels. Organic thin film layers of the R, G, and B unitpixels, including R, G, B electroluminescent layers, are formed on theanode electrodes of the R, G and B unit pixels, respectively, in theapertures. A semitransmissive cathode electrode is formed on the entiresurface of the substrate.

The front surface emitting organic electroluminescent display has formedthereon anode electrodes with a 2-layered structure, employing a firstanode electrode as a metal film having a high reflectivity, and a secondanode electrode as a metal film capable of conforming with a workfunction, thereby improving luminous efficiency by increasingreflectivity and a hole injecting characteristic.

However, in the front surface emitting organic electroluminescentdisplay, all of the second anode electrodes of the respective R, G and Bunit pixels have the same thickness. Therefore, it is impossible toobtain desired color reproduction and efficiency, since the first anodeelectrode with a good reflectivity and the semitransmissive cathodeelectrode have respective lengths of optical constructive interferencewhich are different from each other.

SUMMARY OF THE INVENTION

Therefore, to solve the problem described hereinabove, an object of thepresent invention is to provide an organic electroluminescent display,and a method for fabricating the same, capable of obtaining maximumcolor reproduction and highest efficiency.

Another object of the present invention is to provide an organicelectroluminescent display, and a method for fabricating the same,capable of obtaining a desired color reproduction and illuminousefficiency by differently forming the thicknesses of anode electrodes ofrespective R, G and B unit pixels.

Still another object of the present invention is to provide an organicelectroluminescent display, and a method for fabricating the same,capable of improving color reproduction and luminous efficiency by usinga simple process of differently forming the thicknesses of anodeelectrodes of respective R, G and B unit pixels without an additionalmask process.

To accomplish the above-mentioned objects, the present inventionprovides an organic electroluminescent display, comprising: anodeelectrodes of R, G and B unit pixels formed so as to be separated fromeach other on a substrate; organic thin-film layers of the R, G and Bunit pixels formed on the anode electrodes; and a cathode electrodeformed on an entire surface of the substrate; wherein an anode electrodeof at least one unit pixel of the R, G and B unit pixels has a thicknessdifferent from anode electrodes of the other unit pixels.

In an embodiment of the invention, each of the anode electrodes of theunit pixels comprises a first film having a high reflectivity and asecond film for adjusting a work function, and the second film of atleast one unit pixel has a thickness different from the second films ofthe other unit pixels. The second film of the R unit pixel is thickerthan the second films of the other unit pixels.

In a preferred embodiment of the invention, the thickness of the secondfilm of the R unit pixel is in a range from 250 to 450 Å or from 700 to750 Å, and the thicknesses of the second films of the G and B unitpixels are in a range from 50 to 150 Å. The thickness of the second filmof the R unit pixel is in a range from 250 to 450 Å or from 700 to 750Å, the thickness of the second film of the G unit pixel is in a rangefrom 200 to 300 Å, and the thickness of the second film of the B unitpixel is in a range from 50 to 150 Å.

In a further embodiment of the invention, in order to obtain maximumefficiency, in the R, G and B unit pixels, the thickness of the secondfilm of the R unit pixel is 375 Å, the thickness of the second film ofthe G unit pixel is 250 Å, and the thickness of the second film of the Bunit pixel is 125 Å. Moreover, in order to obtain maximum colorreproduction, the thickness of the second film of the R unit pixel is750 Å, the thickness of the second film of the G unit pixel is 250 Å,and the thickness of the second film of the B unit pixel is 125 Å.

The first film of each of the unit pixels is composed of Al, Ag or analloy film thereof, and the second films are composed of ITO or IZO.

Further, the present invention provides an organic electroluminescentdisplay comprising a number of pixels, each including at least an anodeelectrode, wherein the anode electrodes of adjacent pixels among thenumber of pixels have different thicknesses with respect to each other.

In an embodiment of the invention, the anode electrode of each of thepixels comprises a first film having a high reflectivity and a secondfilm for adjusting a work function, the second films of the anodeelectrodes of adjacent pixels having a thickness different from eachother.

In addition, the present invention provides a method for fabricating anorganic electroluminescent display, comprising the steps of: formingfirst anodes of R, G and B unit pixels on a substrate; forming an anodeelectrode of the R unit pixel by forming a second anode of the R unitpixel on the first anode of the R unit pixel; forming anode electrodesof the G and B unit pixels by forming second anodes of the G and B unitpixels on the first anodes of the G and B unit pixels; forming organicthin-film layers on the anode electrodes of the R, G and B unit pixels,respectively; and forming a cathode electrode on an entire surface ofthe substrate; wherein a second anode of at least one unit pixel, amongthe R, G and B unit pixels, has a thickness different from thethicknesses of the second anodes of the other unit pixels.

Further, the present invention includes a method for fabricating anorganic electroluminescent display, comprising the steps of: formingsequentially a first anode electrode material and a second anodeelectrode material of the R, G and B unit pixels on a substrate; forminganode electrodes of the R, G and B unit pixels, each including a firstanode and a second anode, by etching the first and the second anodeelectrode materials; forming organic thin-film layers on the anodeelectrodes of the R, G and B unit pixels, respectively; and forming acathode electrode on an entire surface of the substrate; wherein asecond anode of at least one unit pixel, among the R, G and B unitpixels, has a thickness different from the thicknesses of second anodesof the other unit pixels.

Further, the present invention includes a method for fabricating anorganic electroluminescent display, comprising the steps of: formingfirst anodes of R, G and B unit pixels on a substrate; forming a secondanode electrode material on an entire surface of the substrate; etchingthe second anode electrode material to form second anodes on the firstanodes of the R, G and B unit pixels, respectively, and to form anodeelectrodes of the R, G and B unit pixels; forming organic thin-filmlayers on the anode electrodes of the R, G and B unit pixels,respectively; and forming a cathode electrode on an entire surface ofthe substrate; wherein a second anode of at least one unit pixel, amongthe R, G and B unit pixels, has a thickness different from thethicknesses of second anodes of the other unit pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate the same or similar components, wherein:

FIG. 1 is a cross-sectional view of a prior art organicelectroluminescent display;

FIG. 2 is a cross-sectional view of an organic electroluminescentdisplay in accordance with an embodiment of the present invention;

FIGS. 3A thru 3E are cross-sectional views of processes for fabricatingan organic electroluminescent display in accordance with a firstembodiment of the present invention;

FIGS. 4A thru 4D are cross-sectional views of processes for fabricatingan organic electroluminescent display in accordance with a secondembodiment of the present invention;

FIGS. 5A thru 5D are cross-sectional views of processes for fabricatingan organic electroluminescent display in accordance with a thirdembodiment of the present invention; and

FIGS. 6A thru 6C are spectrums of R, G and B colors, respectively, in anorganic electroluminescent display in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a detailed description of preferred embodiments of thepresent invention will be apparent in connection with the accompanyingdrawings.

FIG. 1 is a cross-sectional view of a prior art organicelectroluminescent display. As shown in FIG. 1, anode electrodes 110,120 and 130 of the respective R, G and B unit pixels are formed on aninsulating substrate 100. The anode electrodes 110, 120 and 130 includefirst anodes 111, 123, 125 and second anodes 121, 123 and 125,respectively. A pixel defining layer 140 is formed to expose portions ofthe anode electrodes 110, 120 and 130, thereby forming apertures 141,143 and 145 of the respective R, G and B unit pixels. Organic thin filmlayers 150, 160 and 170 of the R, G, and B unit pixels, including R, G,B electroluminescent layers, are formed on the anode electrodes 110, 120and 130 of the R, G and B unit pixels in the apertures 141, 143 and 145,respectively. A semitransmissive cathode electrode 180 is formed on anentire surface of the substrate.

The front surface emitting organic electroluminescent display has anodeelectrodes with a 2-layered structure, employing a first anode electrodeas a metal film having a high reflectivity, and a second anode electrodeas a metal film capable of conforming with a work function, therebyimproving luminous efficiency by increasing reflectivity and a holeinjecting characteristic.

However, in the front surface emitting organic electroluminescentdisplay, all of the second anode electrodes of respective R, G and Bunit pixels have the same thickness. Therefore, it is impossible toobtain desired color reproduction and efficiency, since the first anodeelectrode with a good reflectivity and the semitransmissive cathodeelectrode have a different lengths of optical constructive interferencewith respect to each other

FIG. 2 is a cross-sectional view of an organic electroluminescentdisplay in accordance with an embodiment of the present invention.

Referring to FIG. 2, a buffer layer 210 is formed on a transparentinsulation substrate 200, and semiconductor layers 220, 230 and 240 ofR, G and B unit pixels provided with source/drain regions 221 and 225,231 and 235, and 241 and 245 are formed on the buffer layer 210,respectively. Gates 261, 263 and 265 of the respective unit pixels areformed on a gate insulating film 250, and source/drain electrodes 281and 285, 291 and 295, and 301 and 305 of the respective unit pixelsconnected to the source/drain regions 221 and 225, 231 and 235, and 241and 245 of the respective unit pixels through contact holes (not shown)are formed on an inter-layer insulating film 270.

Further, anode electrodes 320, 330 and 340 of the R, G and B unit pixelsare formed on a planarization film 310 so as to be connected to one ofthe source/drain electrodes of thin-film transistors of the unit pixels,for example, drain electrodes 285, 295 and 305 through via-holes 311,313 and 315, respectively. At this time, each of the anode electrodes320, 330 and 340 of the R, G and B unit pixels includes a first anode321, 331 and 341 having a high reflectivity and a second anode 325, 335and 345 for adjusting a work function, and the second anode of at leastone unit pixel among the R, G and B unit pixels, is formed to have athickness different from the thicknesses of the second anodes of theother unit pixels.

In an embodiment of the present invention, the second anode 325 of the Runit pixel is formed so as to be thicker than the second anodes 335 and345 of the G and B unit pixels, and the thicknesses of the second anodes335 and 345 of the G and B unit pixels are formed with the samethickness. Otherwise, the second anode 325 of the R unit pixel is formedso as to be thicker than the second anodes 335 and 345 of the G and Bunit pixels, and the second anode 335 of the G unit pixel is formed soas to be thicker than the second anode 345 of the B unit pixel.

On the planarization film 310, a pixel defining layer 360 for separatingthe anode electrodes 320, 330 and 340 of the respective unit pixels isformed. The pixel defining layer 360 is provided with apertures 371, 373and 375 for exposing portions of the anode electrodes 320, 330 and 340,respectively. The pixel defining layer 360 may employ a conventionalthermosetting resin or a photosensitive resin. Organic thin-film layers381, 383 and 385 of the respective unit pixels are formed on the anodeelectrodes 320, 330 and 340 of the respective unit pixels in theapertures 371, 373 and 375, and a cathode electrode 390 is formed on anentire surface of the substrate. The organic thin-film layers 381, 383and 385 of the respective unit pixels comprise an electroluminescentlayer of the respective unit pixels, including at least one of a holeinjecting layer, a hole transporting layer, a hole blocking layer, anelectron injecting layer and an electron transporting layer.

The organic electroluminescent display in accordance with the presentinvention is capable of obtaining the highest luminous efficiency byforming the thicknesses of the second anodes 325, 335 and 345 of theanode electrodes 320, 330 and 340, respectively, of the R, G and B unitpixels, respectively, so as to be different from each other dependingupon the unit pixels.

Hereinafter, a detailed description of a method for fabricating anorganic electroluminescent display provided with anode electrodes havinga thickness different from each other, depending upon the R, G and Bunit pixels, in accordance with the present invention, will be provided.In the method for fabricating the organic electroluminescent display inaccordance with the present invention, since the process prior toformation of the anode electrode is similar to a conventional method,hereinafter, only the process of forming the anode electrodes havingthicknesses different from each other, depending upon the R, G and Bunit pixels, and the following processes, will be described.

A description of the method for fabricating the organicelectroluminescent display in accordance with a first embodiment of thepresent invention will be given with reference to FIGS. 3A to 3E.

Referring to FIG. 3A, a first anode electrode material 410 is formed tohave a thickness of 2000 Å, with a metal film such as Al, Ag or an alloythereof having a high reflectivity, on a transparent insulatingsubstrate 400, such as a glass substrate, by using a DC sputter.Referring to FIG. 3B, the first anode electrode material 410 ispatterned to form first anodes 421, 423 and 425 with all of the R, G andB unit pixels having the same thickness.

Referring to FIG. 3C, a second anode electrode material is depositedwith a predetermined thickness on the entire surface of the substratewith a material having a suitable work function, for example, ITO orIZO, and is patterned to form a second anode 431 of the R unit pixel onthe first anode 421 of the R unit pixel only. The second anode 431 ofthe R unit pixel is formed with a thickness of 250˜450 Å or 700˜750 Å,preferably 375 Å.

Referring to FIG. 3D, material the same as the second anode electrodematerial of the R unit pixel (for example, ITO or IZO) is deposited witha thickness of 50˜150 Å on the entire surface of the substrate, and ispatterned to form second anodes 433 and 435 of the G and B unit pixelson the first anodes 423 and 425, respectively. Therefore, the anodeelectrodes 441, 443 and 445 of the R, G and B unit pixels, including thefirst anodes 421, 423 and 435 having a high reflectivity and the secondanodes having suitable work functions 431, 433 and 435, are formed.

On the other hand, by adding a mask process instead of equally formingthe thicknesses of the second anodes of the G and B unit pixels, thesecond anode 433 of the G unit pixel is formed with a thickness of200˜300 Å, preferably 250 Å, and the second anode 435 of the B unitpixel is formed with a thickness of 50˜150 Å, preferably 125 Å, therebyforming the second anodes 431, 433 and 435 of the R, G and B unitpixels, respectively, with different thicknesses from each other.

Referring to FIG. 3E, after the organic insulating film composed of athermosetting resin or a photosensitive resin is deposited on the entiresurface of the substrate, it is patterned by a conventional method toform a pixel defining layer 450 for separating the anode electrodes 441,443 and 445 of the respective unit pixels. After the completion offorming the pixel defining layer 450, the layer 450 is sequentiallycleansed by using water, isopropyl alcohol and acetone. Then, it iscleansed by using a UV/O3 cleanser. At this time, luminescence sectionsof the anode electrodes of the respective unit pixels are opened,depending upon the forming of the pixel defining layer 450, and have apattern dimension of 2 mm×2 mm.

Subsequently, while not shown, in the process of forming the organicthin-film layer on the anode electrodes 441, 443 and 445, correspondingorganic films from among the hole injecting layer, the hole transportinglayer, the electroluminescent layer, the hole blocking layer, and theelectron transporting layer of the R, G and B unit pixels aresequentially formed. The hole injecting layer is formed with a thicknessof 250 Å by using an IDE 406 of Idemitsu Co. with vacuum deposition, andthe hole transporting layer is formed with a thickness of 100 Å byvacuum deposition with a speed of 0.1 nm/sec by usingNBP{N,N′-di(naphthalene-1-yl) -N,N′-diphenyl-benzidine}.

Continuously, CBP {4,4′-bis(carozol-9-yl)-biphenyl} and phosphorescenered are heat deposited with a 100: 12 mixing weight ratio to form theelectroluminescent layer of the R unit pixel with a thickness of 300 Å.The CBP and IrPPy{tris(phenylpyridine)Iridium} are heat deposited with a100:5 mixing weight ratio to form the electroluminescent layer of the Gunit pixel with a thickness of 250 Å. A blue host and a blue dopant areheat deposited with a 100:4 mixing weight ratio to form theelectroluminescent layer of the B unit pixel with a thickness of 150 Å.

Next, BAlq is deposited with a thickness of 50 Åto form a hole barrierlayer, and Alq3 {tri(8-quinolinolate)-aluminium} is vacuum deposited toform the electron transporting layer with a thickness 250 Å. Magnesium(Mg) and silver (Ag) are heat deposited with 10:1 to 30:1 mixing weightratio to form a semitransmissive cathode having a thickness of 50˜150 Å,preferably 100 Å. Then, IZO is deposited using sputter, under a vacuumcondition, of a speed of 0.2 nm/sec and a pressure of 1×10^-5 Pa to forma transmissive cathode electrode.

Finally, a passivation layer is formed to prevent oxygen and moisturefrom penetrating to the exterior, and to protect the inner organicthin-film layer. An encapsulation substrate is attached and encapsulatedby using a UV adhesive under a nitrogen gas atmosphere and an anhydrouscondition. Then, the front surface emitting organic electroluminescentdisplay is manufactured by thermosetting fr 1 hour at a temperature ofabout 70° C.

A description of the method for fabricating the organicelectroluminescent display in accordance with a second embodiment of thepresent invention will be provided with reference to FIGS. 4A to 4D. Thefabricating method in accordance with the second embodiment of thepresent invention is accomplished with the same condition as the firstembodiment, except that the mask process is reduced than the firstembodiment by using a half-tone mask.

Referring to FIG. 4A, a first anode material 510 and a second anodematerial 520 are sequentially deposited on an insulating substrate 500.Referring to FIG. 4B, a photosensitive film 530 is coated on the secondanode electrode material. Then, a photo process is performed by usingthe half-tone mask 540. The half-tone mask 540 includes: a lightblocking section 541 for blocking light entirely, corresponding to aportion forming the anode electrode of the R unit pixel;halftransmitting sections 543 and 545 for transmitting a portion oflight, corresponding to portions forming the anode electrodes of the Gand B unit pixels; and a transmitting section 547 for transmitting lightentirely.

Referring to FIG. 4C, photosensitive film patterns 531, 533 and 535having thicknesses different from each other, depending upon the R, Gand B unit pixels, are formed by a photo process using the half-tonemask 540. The photosensitive film pattern 531 of the R unit pixel isformed so as to be thicker than the photosensitive film patterns 533 and535 of the G and B unit pixels, and the photosensitive film patterns 533and 535 of the G and B unit pixels have the same thickness.

Referring to FIGS. 4C and 4D, the first and the second anode electrodematerials 510 and 520 are patterned by using the photosensitive filmpatterns 531, 533 and 535 to form the anode electrodes 551, 553 and 555having thicknesses different from each other, depending upon the R, Gand B unit pixels. At this time, the first anodes 511, 513 and 515 fromamong the anode electrodes 551, 553 and 555 of the R, G and B unitpixels have the same thickness, and the second anodes 521, 523 and 525have thicknesses different from each other, depending upon the thicknessdifference of the photosensitive film patterns 531, 533 and 535.

That is to say, the second anode 521 of the R unit pixel is formed so asto be thicker than the second anodes 523 and 525 of the G and B unitpixels, and the second anodes 523 and 525 of the G and B unit pixels areformed so as to have the same thickness. In the second embodiment, thesecond anodes 523 and 525 of the G and B unit pixels may be formed so asto have different thicknesses by making the thicknesses of the halftransmitting patterns 543 and 545 of the half-tone mask 540 differentwhen the photosensitive film 530 is patterned using the half-tone maskshown in FIG. 4B.

A description of the method for fabricating the organicelectroluminescent display in accordance with a third embodiment of thepresent invention will be provided with reference to FIGS. 5A to 5D. Thefabricating method in accordance with the third embodiment of thepresent invention is accomplished with the same condition as the firstembodiment, except that the mask process is reduced relative to thefirst embodiment by using a half-tone mask.

Referring to FIG. 5A, a first anode electrode material is deposited andpatterned on an insulating substrate 600 to form the first anodes 611,613 and 615 of the R, G and B unit pixels having the same thickness.Subsequently, the second anode electrode material 620 is deposited onthe substrate 600 including the first anodes 611, 613 and 615.

Referring to FIG. 5B, a photosensitive film 630 is coated on the secondanode electrode material. Then, a photo process is performed by usingthe half-tone mask 640. The half-tone mask 640 includes: a lightblocking section 641 for blocking light entirely, corresponding to afirst anode 611 of the R unit pixel; half transmitting sections 643 and645 for transmitting a portion of light, corresponding to second anodes613 and 615 of the G and B unit pixels; and a transmitting section 647for transmitting light entirely.

Referring to FIG. 5C, photosensitive film patterns 631, 633 and 635having thicknesses different from each other, depending upon the R, Gand B unit pixels, are formed by a photo process using the half-tonemask 640. The photosensitive film pattern 631 of the R unit pixel isformed so as to be thicker than the photosensitive film patterns 633 and635 of the G and B unit pixels, and the photosensitive film patterns 633and 635 of the G and B unit pixels have the same thickness.

Referring to FIGS. 5C and 5D, the second anode electrode material 620 ispatterned by using the photosensitive film patterns 631, 633 and 635 toform the second anodes 621, 623 and 625, respectively, havingthicknesses different from each other depending upon the R, G and B unitpixels. That is to say, the second anode 621 of the R unit pixel isformed so as to be thicker than the second anodes 623 and 625 of the Gand B unit pixels, and the second anodes 623 and 625 of the G and B unitpixels are formed so as to have the same thickness.

Therefore, the anode electrodes 651, 653 and 655 of the R, G and B unitpixels, respectively, are composed of the first anodes 611, 613 and 615,respectively, having the same thickness, and the second anodes 521, 523and 525, respectively, having thicknesses different from each other,thereby having thicknesses different from each other depending upon theR, G and B unit pixels. In the third embodiment, the second anodes 623and 625 of the G and B unit pixels, respectively, may be formed to havedifferent thicknesses by making the thicknesses of the half transmittingpatterns 643 and 645 different from each other when the photosensitivefilm 630 is patterned using the half-tone mask shown in FIG. 5B.

Table 1, Table 2 and Table 3 represent efficiency, brightness andchromaticity coordinates of the R, G and B unit pixels depending uponthe thicknesses of the second anodes in accordance with the presentinvention.

TABLE 1 R unit pixel thickness efficiency brightness chromaticitycoordinates (Å) (Cd/A) (Lm/W) (CIE_x, CIE_y) 125 5.92 3.60 0.62, 0.38375 12.03 7.76 0.64, 0.35 500 0.44 0.19 0.68, 0.31 750 5.59 3.43 0.67,0.33

TABLE 2 G unit pixel thickness efficiency brightness chromaticitycoordinates (Å) (Cd/A) (Lm/W) (CIE_x, CIE_y) 125 32.33 17.26 0.23, 0.68375 10.85 4.85 0.45, 0.53 500 0.23 0.06 0.32, 0.40 750 3.20 1.37 0.52,0.47

TABLE 3 B unit pixel thickness efficiency brightness chromaticitycoordinates (Å) (Cd/A) (Lm/W) (CIE_x, CIE_y) 125 4.24 2.81 0.13, 0.14375 3.28 1.95 0.21, 0.49 500 0.17 0.07 0.18, 0.08 750 1.46 0.73 0.33,0.53

From Table 1, the R unit pixel has the highest efficiency and brightnesswhen the thickness is 375 Å, and the chromaticity coordinates has thehighest value when the thickness is 750 Å. Therefore, considering all ofthe efficiency, brightness and chromaticity coordinates, it ispreferable that the second anode of the anode electrode in the R unitpixel be formed with a thickness of 375 Å.

From Table 2, since the G unit pixel has the highest efficiency andbrightness when the thickness is 125 Å, and the chromaticity coordinatesare also stable, it is preferable that the second anode of the anodeelectrode in the G unit pixel be formed with a thickness of 125 Å.

From Table 3, since the B unit pixel has the highest efficiency andbrightness when the thickness is 125 Å, and the chromaticity coordinatesare also stable, it is preferable that the second anode of the anodeelectrode in the B unit pixel be formed with a thickness of 125 Å.

FIGS. 6A to 6C are spectrums of R, G and B, respectively, in an organicelectroluminescent display in accordance with the present invention.

In accordance with the embodiment of the present invention, as thethicknesses of the electrodes for conforming the work function among theanode electrodes of the multi-layered structure are formed differentlyfrom each other depending upon the R, G and B unit pixels, therespective unit pixels are capable of obtaining the highest efficiency.Further, the R and B unit pixels are capable of obtaining both thehighest efficiency and the maximum color purity when a full color deviceis embodied.

In addition, when the anode electrodes having different thicknesses areformed, since an additional process is excluded by adopting thehalf-tone mask, effects of a process simplification and yieldimprovement are produced.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment, but on the contrary, it is intended to covervarious modification within the spirit and the scope of the appendedclaims.

1. A method for fabricating an organic electroluminescent display,comprising the steps of: disposing first anodes of red, green and blueunit pixels on a substrate; forming an anode electrode of the red unitpixel by disposing a second anode of the red unit pixel on the firstanode of the red unit pixel, each anode electrode of the red unit pixelincluding a first film having a high reflectivity and a second film foradjusting a work function; forming anode electrodes of the green andblue unit pixels by disposing second anodes of the green and blue unitpixels on the first anodes of the green and blue unit pixels,respectively, each anode electrode of the green and blue unit pixelsincluding a first film having a high reflectivity and a second film foradjusting a work function; disposing respective organic thin-film layerson the anode electrodes of the red, green and blue unit pixels; anddisposing a cathode electrode over an entire surface of the substrate;wherein the second anode of at least one unit pixel of the red, greenand blue unit pixels has a thickness different from thicknesses of thesecond anodes of other unit pixels of the red, green and blue unitpixels; and wherein a thickness of the second film of the red unit pixelis in a range of one of 250 to 450 Å and 700 to 750 Å, a thickness ofthe second film of the green unit pixel is in a range of one of 50 to150 Å and 200 to 300 Å, and a thickness of the second film of the blueunit pixel is in a range of 50 to 150 Å.
 2. A method for fabricating anorganic electroluminescent display, comprising the steps of: disposingsequentially a first anode electrode material and a second anodeelectrode material of red, green and blue unit pixels on a substrate;etching the first and second anode electrode materials to form anodeelectrodes of the red, green and blue unit pixels, each of the anodeelectrodes of the red, green and blue unit pixels including a first filmhaving a high reflectivity and forming a first anode and a second filmfor adjusting a work function and forming a second anode; disposingrespective organic thin-film layers on the anode electrodes of the red,green and blue unit pixels; and disposing a cathode electrode over anentire surface of the substrate; wherein the second anode of at leastone unit pixel of the red, green and blue unit pixels has a thicknessdifferent from thicknesses of the second anodes of other unit pixels ofthe red, green and blue unit pixels; wherein the first and the secondanode electrode materials are patterned by using photosensitive filmpatterns having thicknesses different from each other, depending uponthe red, green and blue unit pixels; and wherein a thickness of thesecond anode of the red unit pixel is in a range of one of 250 to 450 Åand 700 to 750 Å, a thickness of the second anode of the green unitpixel is in a range of one of 50 to 150 Å and 200 to 300 Å, and athickness of the second anode of the blue unit pixel is in a range of 50to 150 Å.
 3. The method according to claim 2, wherein the second anodeof the red unit pixel is thicker than the second anodes of the otherunit pixels.
 4. The method according to claim 2, wherein thephotosensitive film patterns are formed by a photo process using ahalf-tone mask.
 5. A method for fabricating an organicelectroluminescent display, comprising the steps of: disposing firstanodes of red, green and blue unit pixels on a substrate; disposing asecond anode electrode material over an entire surface of the substrate;etching the second anode electrode material to form respective secondanodes on the first anodes of the R, G and B unit pixels, therebyforming respective anode electrodes of the red, green and blue unitpixels, each of the anode electrodes of the red, green and blue unitpixels including a first film having a high reflectivity and a secondfilm for adjusting a work function; disposing organic thin-film layerson the respective anode electrodes of the red, green and blue unitpixels; and disposing a cathode electrode over an entire surface of thesubstrate; wherein a second anode of at least one unit pixel of the red,green and blue unit pixels has a thickness different from thicknesses ofsecond anodes of other unit pixels of the red, green and blue unitpixels; and wherein a thickness of the second film of the red unit pixelis in a range of one of 250 to 450 Å and 700 to 750 Å, a thickness ofthe second film of the green unit pixel is in a range of one of 50 to150 Å and 200 to 300 Å, and a thickness of the second film of the blueunit pixel is in a range of 50 to 150 Å.